1. Field of the Invention
The present invention relates to a circuit and method for generating a pulse signal, particularly to a circuit and method for generating a continuous pulse signal.
2. Description of the Related Art
Refer to FIG. 1 a diagram schematically showing the circuit of a conventional PWM (Pulse Width Modulation) device. The conventional PWM device 10 comprises: a comparator 12, a comparator 14 and a ramp signal generator 16, wherein the comparators 12 and 14 are coupled to each other, and the ramp signal generator 16 is coupled to the comparators 12 and 14. Refer to FIG. 2. The comparator 12 receives a high DC reference voltage V+ and a low DC reference voltage V− and outputs a clock signal. The ramp signal generator 16 then receives the clock signal and outputs a ramp signal to the comparators 12 and 14.
When the value of the ramp signal reaches the high DC reference voltage V+, the comparator 12 outputs a low-level clock signal to the ramp signal generator 16 to change the direction of the ramp signal, and the ramp signal begins to descend. When the value of the ramp signal reaches the low DC reference voltage V−, the comparator 12 outputs a high-level clock signal to the ramp signal generator 16 to change the direction of the ramp signal, and the ramp signal begins to ascend. Thus, the ramp signal is confined to between the high DC reference voltage V+ and the low DC reference voltage V−. Besides, the ramp signal is not necessarily the triangular waveform shown in FIG. 2 but may be a sawtooth waveform.
The comparator 14 receives the ramp signal and an input signal Sin and compares them to output a PWM signal. Refer to FIG. 3A. When the value of the input signal Sin is at the half of the ramp signal, the PWM signal is a pulse train having a 50% duty cycle. Refer to FIG. 3B. When the value of the input signal Sin is at between the high DC reference voltage V+ and the half of the ramp signal, the PWM signal is a pulse train having a duty cycle less than 50%. Refer to FIG. 3C. When the value of the input signal Sin is over the high DC reference voltage V+, the PWM signal is completely a low-level signal. However, such a phenomenon is abnormal. In other words, the conventional PWM device 10 has an intrinsic disadvantage: once the input signal Sin is over the range, the pulse signal is interrupted.
Accordingly, the present invention proposes a circuit and method for generating a continuous pulse signal to solve the abovementioned problem.